Traditionally, shoes contain, in the upper portion of the vamp, a slit which extends visibly into the area corresponding to the instep, and a device for securing the shoe to the foot by bringing together the edges of said slit, for example by the use of a lacing arrangement, in order to bring about the superposition of the vamp over the foot. This type of shoe construction is relatively adaptable to different morphologies of the foot, because it facilitates the regulation of the amount of space between the edges of the slit, and, most especially, because it allows the shoe, for any particular size, to fit several sizes of insteps; it is evident that, as the size of the foot decreases, the metatarsus has progressively greater range of free movement within the portion of the vamp enclosing it, which is not affected by bringing together the edges of the slit that work together with the lacing. It also becomes apparent that the shoe vamp bends and/or loses it shape to a progressively greater extent at the point where it joins the closed end of the slit, as the tight fit over the instep and the free movement of the metatarsus in relation to shoe size become more pronounced. The solution to these problems would obviously entail molding each shoe on the shape and size of each foot, a utopian measure given the current industrial and economic context. On the other hand, different well-known solutions have been devised, consisting, for example, in the standardization of shoe sizes on the basis of increments of length, variable by country, of 6.6 mm for French sizes and of 8.46 mm for American sizes. Proposals have also been made for shoes with adjustable sizes and/or shoes equipped with foot-anchoring devices which are adjustable on the basis of foot size and which are contained within the vamp. The shoes described in U.S. Pat. Nos. 2,112,052 and 1,633,413 may be cited as examples, these patents concerning, respectively, a shoe which is adjustable in length according to the relative movement of the heel in relation to the extremity of the shoe, and a shoe whose internal support structure can be shifted longitudinally in relation to the vamp as a function of foot length. Shoes of this kind have proved to involve the use of complicated methods for allowing the adjustment of length, and have shown themselves to be poorly adapted for uses such as running or walking.
Other types of shoes, such as those described in the French Pat. Nos. 2,541,093 and 2,534,459, have devices for anchoring the foot that are adjustable and are contained within the vamp, which, in itself, performs the sole function of covering the foot. In these shoes, the yokes, designed to ensure the stability of the foot, extend symmetrically in relation to the foot into the area corresponding substantially to the position of the extremities of the first and fifth metatarsals. Thus, by bringing together the free ends of these yokes, the anchoring of the foot onto the sole and, in particular, its longitudinal stability exercised in the area of the metatarsus are ensured. German Pat. No. 399,491 may also be mentioned as an example, since it describes a shoe equipped, first, with an interior securing arrangement designed to adjust an interior supporting tongue for the plantar arch of the foot, and, second, with a device for tightening the vamp over the foot. The combination of such methods for holding the foot firmly in place also does not permit optimal tightening of the metatarsus for the smallest foot sizes included in the range of size increments for a shoe under consideration.